This invention relates to a method for purifying polysaccharides present in the form of an aqueous solution.
A known method for purifying polymers present as solutes in solutions include the step of precipitating the polymers through the addition of auxiliary solvents which are miscible with the solvent components of the polymer solutions but in which the polymers are insoluble. This method has been used in the purification of xanthan gum which is a polysaccharide obtained through fermentation by a microorganism. See for example, U.S. Pat. No. 5,340,743. According to this method, the xanthan gum is precipitated and separated by admixing the culture medium containing xanthan gum with isopropanol as a hydrophilic solvent. This method, may however, require a large amount of isopropanol, making it difficult to recover the isopropanol after the precipitation and separation of the xanthan gum. In addition, the equipment and facilities for carrying out the process are expensive. Moreover, the xanthan gum is precipitated in the form of strings and if a stirring machine is used, it is difficult to perform the purification operation since the xanthan gum coils around the stirring machine.
A method which obviates the above-described difficulties is described and claimed in U.S. Pat. No. 5,315,003, the contents of which are incorporated herein by reference. An aqueous solution Of a polysaccharide and a hydrophilic organic solvent which does not solubilize the polysaccharide are continuously and simultaneously admixed in a rotary turbine to precipitate the polysaccharide. The precipitated polysaccharide is continuously cut with an interlocking cutter comprising a fixed cutter element and a rotary cutter element. A suspension of finely cut polysaccharide particles is continuously produced at the end of the process.
In the apparatus for implementing this continuous polysaccharide precipitation and cutting process, the rotary cutter is driven by the same shaft as the precipitation turbine. The streams of the aqueous polysaccharide solution and the hydrophilic organic solvent are fed by pumps such as rotary pumps or diaphragm pumps to the turbine. With this apparatus, a problem arises in quality control. The polysaccharide particles can have greatly varying sizes, depending on the relative flow rates of the aqueous polysaccharide solution and the hydrophilic organic solvent fed to the turbine.